Due to the limited radio spectrum available for wireless communications, and the need to conserve battery power within mobile stations (MSs), such as cellular phones and terminals, certain networks have implemented a discontinuous reception (DRX) or “slotted mode” protocol. A mobile station in the slotted mode monitors a paging channel of the network at only predetermined times, or slots, within a slot cycle. At all other times in the slot cycle, the mobile station need not monitor the paging channel. This allows the mobile station to operate in a reduced power consumption mode and prolong standby time for a given battery charge.
The network tracks slot times for each mobile station within each geographically based network cell. When a phone call, for example, is routed through the network to a recipient MS, a base station(s) (BS) associated with the appropriate cell(s) transmits a ‘page’ over the paging channel to the recipient MS. Because both the network and the mobile station know which slot the mobile station will be monitoring, the network sends the ‘page’ at a time coincident with or overlapping the monitored slot, rather than continuously over the paging channel. The paging channel is typically a uni-directional downlink (BS to MS) channel. The mobile station uses a different channel (typically a uni-directional uplink channel called Access channel) that does not operate on the slot times of the paging channel to request a dedicated traffic channel for the pending phone call.
In CDMA IS95 or IS2000 versions A, B, and C, the slot cycle is divided into 16 slots of 80 milliseconds each, numbered 0–15. The basic slot cycle time is 1.28 seconds (16 slots at 80 milliseconds each). The frequency of slots monitored by the mobile station may vary according to demands on the network at any given time, such that the current slot cycle time T=1.28×2i seconds, wherein i is an integer 0, 1, 2, etc., known as the slot cycle index. Thus for a slot cycle index i=0, the slot cycle time T is the basic slot cycle, 1.28 seconds, and each mobile station must monitor one slot per each basic cycle. For a slot cycle index i=2 the slot cycle time T is expanded to 5.12 seconds, and each mobile station must monitor only one slot for each 4 basic slot cycles. Typically, the network transmits a maximum slot cycle index that it will support at a particular cell, and each mobile station operates on its maximum slot cycle index consistent with that broadcast maximum (e.g., if a network/cell transmits maximum i=2, an MS that support i≦4 will operate within that network/cell at i=2; if the network/cell transmits maximum i=2, an MS that support i≦1 will operate within that network/cell at i=1).
Mobile stations may employ one of at least two architectures for ‘waking up’ the MS when originating a call. As used herein, the mobile station is in a ‘sleep’ mode when it is powered on but at a power consumption rate that maximizes standby time. The MS is in an ‘awakened’ mode when additional circuitry is powered as compared to the sleep mode (e.g., the MS is monitoring the paging channel). One approach ‘wakes’ the mobile station every 1.28 seconds regardless of slot cycle index (though wakeups between designated slots for i>0 are of a shorter duration). This approach is referred to herein as the fixed interval system.
An alternate approach is to only ‘wake’ the mobile station at the designated paging slot that the network requires the mobile station to monitor, hereinafter, a true index system. Because the true index system wakes the mobile station less often and for less total time, it consumes approximately 10% less power than the fixed interval system at a slot cycle index i=2. Power conservation remains an important consideration in most aspects of mobile station design and operation.
The mobile station must monitor the paging channel only during the slots designated by the network, and the network architecture or protocol does not tie call origination from the mobile station to its assigned slot. Nevertheless, some, if not all, mobile station architectures prevent the mobile station from transmitting a call setup or channel access request to the network until the mobile station wakes up according to its slot schedule. The greatest time lag occurs when the user presses the ‘talk’ or ‘send’ button immediately after the mobile station goes into sleep mode. For the fixed interval approach, this is a delay of 1.28 seconds regardless of slot time index. However, for the true index approach, there is a 5.12 second delay (minus 80 milliseconds) at index i=2, a wait sufficiently long so has to be noticeable by the user.